Re: end to end error recovery musings

[Jeff Garzik <jeff@garzik.org>]

Theodore Tso wrote:
> Can someone with knowledge of current disk drive behavior confirm that
> for all drives that support bad block sparing, if an attempt to write
> to a particular spot on disk results in an error due to bad media at
> that spot, the disk drive will automatically rewrite the sector to a
> sector in its spare pool, and automatically redirect that sector to
> the new location.  I believe this should be always true, so presumably
> with all modern disk drives a write error should mean something very
> serious has happend.  


This is what will /probably/ happen.  The drive should indeed find a 
spare sector and remap it, if the write attempt encounters a bad spot on 
the media.

However, with a large enough write, large enough bad-spot-on-media, and 
a firmware programmed to never take more than X seconds to complete 
their enterprise customers' I/O, it might just fail.


IMO, somewhere in the kernel, when we receive a read-op or write-op 
media error, we should immediately try to plaster that area with small 
writes.  Sure, if it's a read-op you lost data, but this method will 
maximize the chance that you can refresh/reuse the logical sectors in 
question.

	Jeff